Prokaryote - Wikipedia, the free encyclopedia
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Prokaryote
From Wikipedia, the free encyclopedia
Prokaryotes (pro-KAR-ee-oht) (from Old Greek pro- before + karyon nut or kernel, referring to the cell nucleus, + suffix -otos, pl.
-otes; also spelled "procaryotes") are organisms without a cell nucleus (= karyon), or indeed any other membrane-bound organelles, in
most cases unicellular (in rare cases, multicellular).
Contents
1 Relationship to eukaryotes
2 Genes
3 Colonies
4 Structure
5 Environment
6 Evolution of prokaryotes
7 References
8 See also
9 External links
Prokaryotic bacteria cell
structure
Relationship to eukaryotes
Prokaryotes are very distinct from eukaryotes (meaning true kernel, also spelled "eucaryotes") because eukaryotes have true nuclei
containing their DNA, while prokaryotes' genetic material is not membrane-bound. Eukaryotes are organisms that have cell nuclei and
may be variously unicellular or multicellular. An example of an eukaryote would be a human. The difference between the structure of
prokaryotes and eukaryotes is so great that it is considered to be the most important distinction among groups of organisms. Most
prokaryotes are bacteria, and the two terms are often treated as synonyms. In 1977, Carl Woese proposed dividing prokaryotes into the
Bacteria and Archaea (originally Eubacteria and Archaebacteria) because of the significant genetic differences between the two. This
arrangement of Eukaryota (also called "Eukarya"), Bacteria, and Archaea is called the three-domain system replacing the traditional
two-empire system.
The cell structure of prokaryotes differs greatly from eukaryotes in many ways. The defining characteristic is the absence of a nucleus or
nuclear envelope. Prokaryotes also were previously considered to lack cytoskeletons and do lack membrane-bound cell compartments
such as vacuoles, endoplasmic reticulum/endoplasmic reticula, Golgi apparatus, mitochondria, and chloroplasts. In eukaryotes, the latter
two perform various metabolic processes and are believed to have been derived from endosymbiotic bacteria. In prokaryotes similar
processes occur across the cell membrane; endosymbionts are extremely rare. Prokaryotes also have cell walls, while some eukaryotes,
particularly animals, do not. Both eukaryotes and prokaryotes have structures called ribosomes, which produce protein. Prokaryotes are
usually much smaller than eukaryotic cells.
Taking into account membrane bound organelles, prokaryotes also differ from eukaryotes in that they contain only a single loop of DNA
stored in an area named the nucleoid. The DNA of the eukaryote is found on chromosomes. Prokaryotic DNA also lacks the proteins
found in eukaryotic DNA. Prokaryotes have a larger surface area to volume ratio. This gives the Prokaryotes a higher metabolic rate, a
higher growth rate and thus a smaller generation time as compared to the Eukaryotes.
Genes
Nearly all prokaryotes have a single circular chromosome contained within a conglomeration of ribosomes and other proteins related to
transcription and translation region called the nucleoid, as opposed to the well defined, double membrane bound eukaryotic nucleus.
Certain exceptions do apply, however. For example, Borrelia burgdorferi and the genus Streptomyces contain linear chromosomes, like
the eukaryotes. Vibrio cholerae, the causative agent of cholera, has two circular chromosomes, the smaller of which contains most of the
genes responsible for virulence.
Most notable, however, are the plasmids, which are small (about 1 to 10 thousand base pairs), circular pieces of DNA that are replicated
by the host's DNA replication machinery, but whose genes are not absolutely critical for general survival. In nature, they usually contain
special genes that confer some type of selective advantage such as antibiotic resistance, virulence, or gene transfer mechanisms. In
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genetic engineering artificially introduced plasmids carry genes to be expressed and studied.
Prokaryotes also differ from eukaryotes in the structure, packing, density, and arrangement of their genes on the chromosome.
Prokaryotes have incredibly compact genomes compared to eukaryotes, mostly because prokaryote genes lack introns and large
non-coding regions between each gene. Whereas nearly 95% of the human genome does not code for proteins or RNAs or includes a
gene promoter, nearly all of the prokaryote genome codes or controls something. Prokaryote genes are also expressed in groups, known
as operons, instead of individually, as in eukaryotes. In a prokaryote cell, all genes in an operon(three in the case of the famous lac
operon) are transcribed on the same piece of RNA and then made into separate proteins, whereas if these genes were native to
eukaryotes, they each would have their own promoter and be transcribed on their own strand of mRNA. This lesser degree of control
over gene expression contributes to the simplicity of the prokaryotes as compared to the eukaryotes. It is worth noting that one of the
most convincing pieces of evidence for the endosymbiotic theory of the origin of mitochondria is that mitochondrial genomes look like
prokaryotic genomes, replete with circular genomes, operons, and plasmids, while that of the host follows the eukaryotic model.
Reproduction is most often asexual, through binary fission, where the chromosome is duplicated and attaches to the cell membrane, and
then the cell divides in two. However, they show a variety of parasexual processes where DNA is transferred between cells, such as
transformation and transduction.
Colonies
While prokaryotes are nearly always unicellular, some are capable of forming groups of cells called colonies. Unlike many eukaryotic
multicellular organisms, each member of the colony is undifferentiated and capable of free-living (but consider cyanobacteria, a very
successful prokaryotic group which does exhibit definite cell differentiation). Individuals that make up such bacterial colonies most often
still act independent of one another. Colonies are formed by organisms that remain attached following cell division, sometimes through
the help of a secreted slimy layer.
Structure
Recent research indicates that all prokaryotes actually do have cytoskeletons albeit
more primitive than those of eukaryotes. Besides homologues of actin and tubulin
(MreB and FtsZ) the helically arranged building block of flagellum, flagellin is one
of the most significant cytoskeletal protein of bacteria as it provides structural
backgrounds of chemotaxis, the basic cell physiological response of bacteria. At
least some prokaryotes also contain intracellular structures which can be seen as
primitive organelles. Membranous organelles (a. k .a. intracellular membranes) are
known in some groups of prokaryotes, such as vacuoles or membrane systems
devoted to special metabolic properties, e. g. photosynthesis or chemolithotrophy.
Additionally, some species also contain protein-enclosed microcompartments
mostly associated with special physiological properties (e. .g. carboxysomes or gas
vacuoles).
The sizes of prokaryotes relative to other organisms
and biomolecules.
Environment
Prokaryotes are found in nearly all environments on earth. Archaea in particular
seem to thrive in harsh conditions, such as high temperatures or salinity. Organisms such as these are referred to as extremophiles. Many
prokaryotes live in or on the bodies of other organisms, including humans.
Evolution of prokaryotes
It is generally accepted that the first living cells were some form of prokaryote and may have developed out of protobionts. Fossilized
prokaryotes approximately 3.5 billion years old have been discovered, and prokaryotes are perhaps the most successful and abundant
organism even today. In contrast the eukaryote only appeared between approximately 1.7 and 2.2 billion years ago [1]. While Earth is the
only known place where prokaryotes exist, some have suggested structures within a Martian meteorite should be interpreted as fossil
prokaryotes; this is open to considerable debate and skepticism.
Prokaryotes diversified greatly throughout their long existence. The metabolism of prokaryotes is far more varied than that of eukaryotes,
leading to many highly distinct types of prokaryotes. For example, in addition to using photosynthesis or organic compounds for energy
like eukaryotes do, prokaryotes may obtain energy from inorganic chemicals such as hydrogen sulfide.
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This has enabled the bacteria to thrive and reproduce. Today, archaebacteria can be found in the cold of Antarctica and in the hot
Yellowstone springs.
References
1. ^ Scientific American, October 21, 1999
(http://www.sciam.com/askexpert_question.cfm?articleID=000C32DD-60E1-1C72-9EB7809EC588F2D7&catID=3&topicID=3)
See also
Monera – previously Prokaryota were a Kingdom with divisions of eubacteria and archaebacteria.
Bacterial cell structure
Nanobe
Virus
Prion
Protist
Symbiogenesis
External links
This article contains material from the Science Primer (http://www.ncbi.nlm.nih.gov/About/Primer) published by the NCBI, which, as a
US government publication, is in the public domain.
Retrieved from "http://en.wikipedia.org/wiki/Prokaryote"
Category: Prokaryotes
This page was last modified 23:49, 5 February 2007.
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